Heretofore, a fuel cell system has broadly been known where a hydrogen off-gas discharged from a fuel cell meets a hydrogen gas discharged from a high pressure hydrogen tank to be supplied to the fuel cell again. In this fuel cell system, halfway along a supply path extending from the high pressure hydrogen tank to the fuel cell, a circulation path for returning the hydrogen off-gas to the supply path is connected.
In general, since water is produced by an electrochemical reaction in the fuel cell, the hydrogen off-gas contains much water vapor. Therefore, below a freezing point after stopping the system, the water vapor in the hydrogen off-gas is condensed in the circulation path and the like, and condensed water might freeze. To solve the problem, in a fuel cell system disclosed in Japanese Patent Application Laid-Open No. 2005-93110, a pressure in the circulation path or the like is raised after stopping the system, whereby a solidifying point of water is lowered to inhibit the freezing in the circulation path.